1. Field of the Invention
The present invention relates to a phase shift mask of attenuation type and a manufacturing method thereof, and in particular to a structure of a pattern formed on a phase shift mask of attenuation type.
2. Description of the Background Art
Recently, semiconductor integrated circuits have been highly integrated and miniaturized. Accordingly, minituarization of circuit patterns formed on a semiconductor substrate has been rapidly developed. Photolithography is, among others, a widely-known basic technique for forming a circuit pattern. Although various developments and improvements have been made, a circuit pattern has been miniaturized at even higher rate, resulting in an ever-increasing demand for improvement in resolution of a circuit pattern.
Generally, a resolution limit R (nm) in photolithography using a demagnification exposure method is expressed by: EQU R=K.sub.1 .multidot..lambda./(NA) (1)
where .lambda. represents a wavelength (nm) of light used, NA represents a numerical aperture of a lens, and K.sub.1 represents a constant depending on a resist process.
As can be seen from the equation (1), values of K.sub.1 and .lambda. should be made smaller and a value of NA should be made larger to improve a resolution limit. In other words, the constant depending on a resist process should be made smaller, with the wavelength being made shorter and NA being increased. It is technically difficult, however, to improve a light source or a lens, and there is such a problem that depth of focus of light .DELTA. (.DELTA.=k.sub.1 .multidot..lambda./(NA).sup.2) becomes shallower due to a shorter wavelength and a higher NA, thereby causing a drop in resolution in practice.
Now, with reference to FIG. 32, a cross section of a photomask, intensity of amplitude of exposure light on the photomask, and light intensity of the exposure light on a wafer when conventional photomask is used will be described.
Referring to FIG. 32(a), a cross sectional structure of the photomask is described. A metal mask pattern 220 made of a chromium film or the like is formed on a quartz glass substrate 210. Referring to FIG. 32(b), intensity of amplitude of the exposure light on the photomask corresponds to the photomask pattern. Regarding light intensity of the exposure light on the wafer, however, beams of the exposure light transmitting through the photomask are intensified by each other especially when a fine pattern is to be transferred, due to diffraction and interference at adjacent pattern images where beams of light are overlapped, as shown in FIG. 32(c). Consequently, there is only a small difference in light intensities of the beams on the wafer, resulting in poor resolution.
In order to solve this problem, a phase shifting exposure method using a phase shift mask is proposed, for example, in Japanese Patent Laying-Open Nos. 57-62052 and 58-173744.
Referring to FIG. 33, a phase shifting exposure method using a phase shift mask disclosed in Japanese Patent Laying-Open No. 58-173744 will be described. FIG. 33(a) is a cross sectional view of the phase shift mask. FIG. 33(b) shows intensity of amplitude of exposure light on the phase shift mask. FIG. 33(c) shows light intensity of the exposure light on a wafer.
Referring to FIG. 33(a), a phase shift mask 300 has a phase shifter 360 made of a transparent insulating film such as a silicon oxide film and provided at every other opening of a chromium mask pattern 320 formed on a glass substrate 310.
Referring to FIG. 33(b), amplitude intensities on the photomask of beams of the exposure light passing through phase shift mask 310 are inverted alternately by 180.degree.. As a result, in adjacent pattern images, overlapping beams of the exposure light are canceled with each other due to interference of light. Consequently, referring to FIG. 33(c), there is a sufficient difference in light intensities of beams of the exposure light on the wafer, thereby improving resolution of a pattern image.
However, although the above-described shift mask is very effective for a periodic pattern such as lines and spaces, arrangement of a phase shift mask is extremely difficult for a complex pattern. Therefore, it cannot be used for every type of pattern.
In order to solve the above-mentioned problem, an exposure method using a phase shift mask of attenuation type is disclosed in, for example, JJAP Series 5, Proceedings of 1991 International MicroProcess Conference pp.3-9 and Japanese Patent Laying-Open No. 4-136854. The exposure method using a phase shift mask of attenuation type disclosed in Japanese Patent Laying-Open No. 4-136854 will be described below.
FIG. 34(a) is a cross sectional view of the above-mentioned phase shift mask 400 of attenuation type. FIG. 34(b) shows intensity of amplitude of exposure light on the phase shift mask of attenuation type. FIG. 34(c) shows light intensity of the exposure light on a wafer.
Referring to FIG. 34(a), phase shift mask 400 includes a quartz substrate 410 for transmitting exposure light, a light transmitting portion 430 formed on a main surface of quartz substrate 410 for exposing the main surface of quartz substrate 410, and a phase shifter portion 420 for converting phase of the exposure light passing therethrough by 180.degree. with respect to that of the exposure light passing through light transmitting portion 430.
Phase shifter portion 420 has a two-layered structure consisting of a chromium layer 420a having a transmittance of 5-20% with respect to that of light transmitting portion 430, and a shifter layer 420b converting a phase of the exposure light passing therethrough by 180.degree. with respect to that of the exposure light passing through light transmitting portion 430.
Intensity of amplitude on the photomask of the exposure light passing through phase shift mask 400 having such a structure as described above is shown in FIG. 34(b). Since phase of exposure light is inverted at an edge of an exposure pattern, light intensity on the wafer is always 0 at the edge of the exposure pattern as shown in FIG. 34(c). As a result, sufficient difference is provided between light intensities of the beams of the exposure light passing through a light transmitting portion 430 and phase shifter portion 420 of the exposure pattern, thereby improving resolution of a pattern image.
The above-described phase shift mask of attenuation type, however, has following problems.
FIG. 35 is a plan view of phase shift mask 400 of attenuation type shown in FIG. 34(a). Light transmitting portion 430 is 0.45 .mu.m.quadrature., and a pitch (P) thereof is 0.9 .mu.m.
Light intensity in a direction of X axis of one light transmitting portion 430 will be described with reference to FIG. 36. In FIG. 36, lithography by an exposure apparatus is performed under the following conditions: NA =0.57, .sigma.=0.4, wavelength of exposure light=i-line (365 nm), difference in phase of phase shifter portion 420 =180.degree.. The figure shows examples where transmittance of phase shifter portion 420 is 0%, 5%, 10%, and 15%.
As can be seen from the figure, with the greater transmittance of phase shifter portion 420, width W of a pattern image (PA) is narrower, and a pattern image becomes clearer.
However, as transmittance increases, a portion (A) where light intensity is high (hereinafter referred to as a side lobe) appears adjacent to pattern image (PA). The side lobe (A) is formed by overlapping first-order diffraction light of pattern image (PA) and exposure light passing through phase shifter portion 420 in a region where the first-order diffraction light is located. First-order diffraction light of the pattern image has phase difference of 180.degree. with respect to the exposure light of pattern image (PA).
Next, intensities of light and amplitude of exposure light at a cross section taken along a line Y--Y of phase shift mask 400 of attenuation type shown in FIG. 35 will be described with reference to FIGS. 37-39.
FIG. 37 shows intensity of amplitude only of the exposure light passing through light transmitting portion 430 at Y--Y cross section. In the figure, arrows A.sub.1 and B.sub.1 show intensities of amplitude of first-order diffraction light. Intensity of amplitude indicated by arrow B.sub.1 is greater because it is formed in a region where intensities of amplitude indicated by arrow A.sub.1 are overlapped.
FIG. 38 shows intensity of amplitude only of exposure light transmitting through phase shifter portion 420 at Y--Y cross section.
FIG. 39 shows light intensity of the exposure light when intensities of amplitude of the exposure light shown in FIGS. 37 and 38 are combined. As can be seen from the figure, a big side lobe B is formed at an intersection of the extension of diagonal lines of light transmitting portions 430. This is the light intensity at a portion where two side lobes A generated by phase shift mask of attenuation type 400 shown in FIG. 35 are overlapped.
Next, referring to FIGS. 40-42, description will be made to exposure of a resist film using phase shift mask of attenuation type 400 providing such exposure light as mentioned above.
Referring to FIG. 40, a positive resist film 460 is formed on a substrate 450. Positive resist film 460 is exposed to light using phase shift mask of attenuation type 400.
Referring to FIG. 41, positive resist film 460 is developed. At resist film 460, in addition to a pattern 430A corresponding to light transmitting portion 430, a side lobe pattern 430B is formed at a position corresponding to a portion of side lobe B, whereby thickness of resist film 460 is decreased.
Referring to FIG. 42, if substrate 450 is etched by using resist film 460 with the decreased thickness, that portion of substrate 450 is undesirably etched. FIG. 43 is a plan view of the substrate etched in such a manner, where an undesirable pattern 464 which corresponds to the side lobe is present between the originally intended patterns 462.
The above-described problem is experienced when each side of light transmitting portion 430 is 0.45 .mu.m. When light transmitting portion 430 is as small as 0.45 .mu.m.quadrature., beams of first-order diffraction light overlap, causing the above-described problem if many light transmitting portions 430 are arranged. However, if each side of light transmitting portion 430 exceeds 1 .mu.m, even only one light transmitting portion gives rise to the above-mentioned problem.
For example, light intensity of first-order diffraction light is approximately 12% when the pattern size on the mask is 2.0 .mu.m.quadrature., while light intensity of first-order diffraction light reaches as high as 15% when the mask pattern size is as big as 5.0 .mu.m.quadrature.. A bigger mask pattern has a greater light intensity of first-order diffraction light. Combined with light intensity of exposure light transmitting through the phase shifter portion, the light intensity increases to approximately 30%, and this alone exposes the resist film.
As an example, a mark for measuring error in alignment (box-in-box type) will be described with reference to FIGS. 45 and 46.
A mark for measuring error in alignment in accordance with a box-in-box type has a square opening portion 505 of 15 .mu.m.quadrature. formed at a first layer 500 and a square pattern 510 of 5 .mu.m.quadrature. formed of a second layer in opening portion 505.
If square pattern 510 is formed at the very center of opening portion 505, then X.sub.1 =X.sub.2, and Y.sub.1 =Y.sub.2, and thus there is no deviation between the first and second layers. However, if X.sub.1 .noteq.X.sub.2 and Y.sub.1 .noteq.Y.sub.2, deviations in X and Y directions .DELTA.x=(X.sub.1 -X.sub.2)/2, .DELTA.y=(Y.sub.1 -Y.sub.2)/2 are measured, and the error in aligning first and second layers is determined.
Referring to FIG. 47, a phase shift mask of attenuation type 600 used for patterning opening portion 505 at first layer 500 has a phase shifter portion 610 and a light transmitting portion 620 on a substrate. Each side of light transmitting portion 620 is approximately 75 .mu.m.
Referring to FIGS. 48-50, intensities of light and amplitude of transmitting exposure light at a cross section taken along a line S--S of phase shift mask of attenuation type 600 will be described below.
Referring to FIG. 48, intensity of amplitude of exposure light transmitting through light transmitting portion 620 is shown. A big image B.sub.2 formed by first-order diffraction light is seen at the side of a pattern image A.sub.2. FIG. 49 shows intensity of amplitude of exposure light which has past through phase shifter portion 610. Referring to FIGS. 48 and 49, light intensity when beams of exposure light which have passed through light transmitting portion 620 and phase shifter portion 610 are combined is as shown in FIG. 50, where a side lobe B.sub.3 having light intensity of approximately 30-40% is undesirably formed at the side of a pattern image A.sub.3.
Next, formation of opening portion 505 at first layer 500 by using phase shift mask of attenuation type 600 will be described.
Referring to FIG. 51, first layer 500 is formed on a semiconductor substrate 630 and a resist film 650 is formed on first layer 500. Resist film 650 is exposed to light by using phase shift mask of attenuation type 600.
Referring to FIG. 52, resist film 650 is developed and first layer 500 is patterned by using resist film 650. At this time, however, referring to FIG. 53, an undesirable trench 515 is formed at the side of opening portion 505 due to light intensity of side lobe B.sub.3 shown in FIG. 50. Referring to FIG. 54, the undesirable trench 515 is formed almost all around the opening portion 505, resulting in incorrect recognition of an edge of opening portion 505. As a result, measurement of error in alignment cannot be performed correctly.